COLUMN ONE : Learning to Live Without the Bomb : Once at the heart of history, the last of America's nuclear arms designers ponder their future in a nation that no longer needs them.

SCIENCE COMES IN FROM THE COLD. Research in the era of defense conversion . One in an occasional series

He is quick to say he sees no credible nuclear threat for at least the next decade, but in his view that is no reason to give up on weapons design. He sounds wistful when he muses on the possibilities of a new generation of low-yield tactical nuclear weapons that "could take out an aircraft carrier in the Pacific" yet elude detection.

"I don't see any need to build the things," he said, "but we ought to have an active (research) program."

For Thomson, Dearborn, and Livermore itself, the future of such research is embodied in what until recently was one of America's most closely guarded scientific secrets--a tiny pellet shaped like a cold capsule. The pellet, called a hohlraum, is used to trigger nuclear fusion in an experimental process called inertial confinement fusion.

The pellet would be used in a proposed $1.1-billion laser lab, called the National Ignition Facility. When barraged by lasers, the pellet ignites in a fusion reaction that promises a way to examine the titanic forces in a thermonuclear blast without field-testing a bomb.

In October, spirits at Livermore were buoyed when Secretary of Energy Hazel O'Leary approved a proposal to build the test lab there--one of the largest defense and energy projects of the next decade.

But there is no guarantee Congress will fund it.

Livermore scientists already have the NOVA laser--the largest and most powerful laser ever made--to simulate weapons effects and conduct about 300 intricate experiments every year that probe the basic physics of nuclear weapons. But that laser is not powerful enough to simulate fusion bombs. The proposed facility would generate temperatures above 10 million degrees Fahrenheit--the kind of heat created when a hydrogen bomb explodes.

Critics insist that there is no need for the more powerful laser. The National Ignition Facility, they say, is only an expensive sandbox to keep scientists such as Thomson occupied.

"But," countered Thomson, "kids can learn a lot in a sandbox."

(BEGIN TEXT OF INFOBOX / INFOGRAPHIC)

The Bombs They Built

By the best available estimates, the United States has 14,900 nuclear weapons today, compared to 29,000 nuclear warheads in Russia and the other states of the former Soviet Union. The precise number of nuclear weapons, their locations, yields and production histories are still secret.

How Bombs Work:

The atomic bomb starts a fission chain reaction in uranium or plutonium that, in a fraction of a second, unleashes intense radiation and explosive power equal to thousands of tons of dynamite. Today such fission bombs are mostly used as triggers in thermonuclear weapons.

Detonator

Explosive

Casing

Uranium or Plutonium Sphere

Neutron Source

***

The hydrogen bomb uses a fission bomb to compress deuterium and tritium at extremely high temperature to cause instant fusion, producing a blast equal to millions of tons of TNT.

Detonator

Explosive

Casing

Uranium or Plutonium Trigger

Fusion Fuel

Uranium Jacket

Neutron Source

***

How Times Have Changed:

U.S. strategic force levels today, as measured by the number of warheads, are the same as they were in 1972. Russian strategic weapons are believed to be at the same level as they were in 1982. By 2003, the number of U.S. strategic nuclear weapons will be equivalent to those in 1962, just after the Cuban missile crisis. Russian strategic nuclear forces will have returned to their 1976 level.

Bigger Than a Breadbox:

The 100 metric tons of weapons-grade plutonium produced by the United States is so dense that it all could fit in a 6-by-6-foot box.

Trying Them On for Size:

The five declared nuclear powers--the United States, the Soviet Union, Britain, France and China--acknowledge conducting 2,031 nuclear tests since 1945. More than half of those were by the United States.

How Many Is Enough:

Since 1945, the United States has produced about 70,000 nuclear weapons and the Soviet Union is estimated have produced about 55,000.

The Big Bang:

The explosive power of the U.S. nuclear arsenal peaked in 1960 at about 19,000 megatons, or to 19 million kilotons.

Where Old Bombs Go:

The Energy Department's Pantex plant near Amarillo, Tex., is the custodian of more than 6,000 bowling-ball-size "pits" of highly toxic plutonium retrieved from dismantled nuclear weapons.